To detect the presence of an oil film on the sea surface by means of a laser probe beam, two basic techniques have been demonstrated. These techniques are (1) the measurement of fluorescence emission from an oil film that has been excited by a probe beam, and (2) the measurement of the absorption by an oil film of the Raman backscatter from the bulk ocean water that has been excited by a probe beam.
There is a close wavelength similarity between the fluorescence spectrum of naturally occurring seawater substances, known as Gelbstoff, and the fluorescence spectrum of oil. This means that the fluorescence spectral signature, by itself, in many cases is not a good means of detecting thin oil films with a high degree of sensitivity. Therefore, a second independent physical measurement is needed. One such measurement technique is the Raman depression technique.
In the Raman depression technique, a pulsed laser beam is directed at the ocean surface. The beam that penetrates into the water interacts with the bulk water and produces Raman backscattering at a shifted wavelength. The difference between the wavelength of the pulsed laser beam and wavelength of light produced by Raman backscattering (Raman wavelength) is called the Raman shift. The magnitude of the return signal at the Raman wavelength is monitored with a receiver sensitive to the Raman wavelength. With uniform water properties, this magnitude is consistent from pulse to pulse. With an oil film on the surface and with the suitably chosen wavelength, for example, in the ultraviolet wavelength range where the absorption coefficient for oil is high, the magnitude of the return signal is noticeably reduced or depressed. This reduction in magnitude is used to detect the presence of the oil film.
The effect of an oil slick on the observed laser Raman backscatter signal that originates in a water column is given by equation (1) EQU S.sub.r =S.sub.r (o)exp-(k.sub.ex +k.sub.r)D (1)
where S.sub.r =magnitude of observed Raman signal
S.sub.r (o)=magnitude of Raman signal without oil slick PA1 k.sub.ex =l absorption coefficient at laser excitation wavelength PA1 k.sub.r =absorption coefficient at Raman wavelength PA1 D=oil slick thickness
Using the methods of the prior art, the difference in magnitude from one surface location to another would indicate the presence of oil on the surface.
Other effects, in addition to the absorption of Raman backscattered light in the oil film being detected, may also cause a decrease in the magnitude of the returned signal and thus introduce system noise. Examples of such effects are atmospheric transmission variations due to clouds, fog patches and haze layers, surface waves and subsurface variations in water attenuation. All of these effects can introduce modulations in the magnitude, indistinguishable from the absorption of the Raman wavelength in the oil film. Thus, these modulations constitute system noise.
Of the above system noise sources, the magnitude modulating action of surface waves is especially important since waves are nearly always present with amplitudes that will result in refractive effects that generate the type of system noise described in the preceding paragraph. This magnitude modulation is a result of the variable curvature of the air/water interface which acts as a dynamic optical lens. Surface waves thereby significantly limit the detectability of surface oil films.
The previous Raman depression technique requires a reference measurement without the oil film at a different time and at a different surface location. This means that measurements include system noise, such as that due to the wave modulation. Since these measurements are independent, the system noise can not be nulled. This limits the minimum detectable film thickness capability of the Raman depression technique.
Thus, there exists a need for a method detecting surface films that does not require independent measurements at two different locations to confirm the presence of an oil slick thereby nulling the effects of system noise.